Assuming you're buying pop under traffic on RON (run of network), setting a high CPM bid could give you access to higher quality websites.
RON traffic in large countries like the US can literally have thousands of websites where your pop under ads are shown. After running some traffic you can see that a good portion of these sites aren't going to be profitable for you, leaving you with a handful of quality sites. To get more traffic on these sites that are working for you, you'd have to increase your bid.
When just starting a new campaign you're bidding against people that have already excluded the low-quality sites from their campaign and have raised their bids on the higher quality sites. To get this higher quality traffic you have to set a higher CPM bid.
RON traffic in large countries like the US can literally have thousands of websites where your pop under ads are shown. After running some traffic you can see that a good portion of these sites aren't going to be profitable for you, leaving you with a handful of quality sites. To get more traffic on these sites that are working for you, you'd have to increase your bid.
When just starting a new campaign you're bidding against people that have already excluded the low-quality sites from their campaign and have raised their bids on the higher quality sites. To get this higher quality traffic you have to set a higher CPM bid.